1. Field of the Invention
The present invention relates to a polyester film for a highly heat sensitive original sheet for stencil printing. In particular, the present invention relates to a film for a highly heat sensitive original sheet for stencil printing, which comprises at least two polyester resins having specific properties, is excellent in a perforation sensitivity and resistance to curl because of its specific shrinking characteristics, and also excellent in resolution and image density of a printed image.
2. Description of the Related Art
Hitherto, as a heat sensitive original sheet for stencil printing, a laminate of a thermoplastic resin film such as a polyester film and a porous thin (tissue) paper sheet. The film to be used in this use is required to have the following properties:
(1) The film has a good handling property and productivity in the preparation of the film itself and the original sheet. That is, in the preparation of the film, the film has good drawability and does not suffer from troubles such as breakage. Further, it should have good winding and slitting properties, so that it is not wrinkled or does not suffer from telescopic winding. In the preparation of the original sheet, it should not cause troubles in an unwinding step or during traveling of the film in the steps.
(2) The film should have sufficient strength and modulus for enduring the lamination works with the porous tissue paper and printing works. The film should not curl due to the lamination condition or change of temperature and humidity during storage. When it curls to a large extent, the handing property and transportability of the original sheet in a stencil printing machine are deteriorated, so that troubles such as clogging of the original sheet will occur.
(3) The film should have good thermal perforation sensitivity. That is, the film is molten with a small amount of heat, and has a shrinking property sufficient for forming suitable size holes to provide a clearly printed image.
(4) In the multiple engravings, the thermal perforation sensitivity is not decreased. That is, a surfactant and a silicone oil in a releasing layer, or molten mass of the film do not adhere to and contaminate a thermal head.
(5) Gradation of the thermal perforation is good. When the original sheet is used, if a part surrounding a specific part to be perforated is molten by the influence of perforation of the specific part, the degradation of the printed image is deteriorated unpreferably. Accordingly, the film should have a thermal perforating property that the part to be perforated and the part to be not perforated are clearly distinguished.
As the films used as a layer of the original sheet for stencil printing, there are known biaxially oriented thermoplastic resin films, thermal properties of which are specified to improve printing characteristics (Japanese Patent KOKAI Publication No. 149496/1987), a surface roughness and the number of protrusions of which are specified (Japanese Patent KOKAI Publication No. 227634/1988) and thermal shrinking properties of which are specified (Japanese Patent KOKAI Publication Nos. 282983/1987, 160895/1988, 312192/1988 and 30996/1991).
However, these films do not necessarily satisfy all the above required properties.